1. Field of the Invention
This invention relates to a magnetic recording medium suitable for high-density magnetic recording, especially for reproducing in use of a magnetoresistive head(MR head).
2. Description of Related Art
In recent years, a recording wavelength tends to be shorter along with high densification, and if the thickness of the magnetic layer is thick, the output tends to be lowered, thereby raising problems in a self-demagnetization loss during recording and a thickness loss during reproducing. Therefore, a magnetic layer is tried to be thinner, but the influence of a nonmagnetic support may easily appear on the surface of the magnetic layer when the magnetic layer of 2 xcexcm or less is directly applied to a support, and a deteriorating tendency in an electromagnetic characteristics or dropouts may be seen.
As a means to solve such problems, there is a method for forming, by using a simultaneous multiplayer coating technique, a nonmagnetic layer on a support and a magnetic layer on the nonmagnetic layer, thereby coating high concentration magnetic coating liquid with a thin thickness as disclosed in Japanese Unexamined Patent Publication (KOKAI) Showa No. 63-191,315 and U.S. Pat. No. 4,863,793. With these inventions, production yields were remarkably improved, and good electromagnetic characteristics were gained. However, a recording and reproducing system with a further narrowed track width has been developed to additionally enhance a recording density. In response to this, it has been proposed that reproduction is made by using a high-sensitive MR head, which has been put to practical use in a hard disk or the like. In a flexible magnetic recording medium such as a magnetic tape, the recording density can be theoretically improved by using the MR head. Even though an output is sufficiently high in conventional magnetic recording media, noise is high and a good C/N is not always gained when the MR head is used. Generally, the surface roughness is reduced to gain a good C/N, but such reduction also causes running stability of the medium to be degraded due to an enlarged contacting area with fixing members such as tape guides or the like used in a magnetic tape path in the recording and reproducing apparatus.
It is an object of the present invention to provide a magnetic recording medium showing a high C/N during high-density magnetic recording and having superior running stability.
The inventors have diligently researched structures of magnetic layers and nonmagnetic layers, and surface roughness of the magnetic layer to accomplish the above object. Consequently, they have found out that the above object could be accomplished by a magnetic recording medium comprising a flexible nonmagnetic support, a nonmagnetic layer containing nonmagnetic powders and a binder formed on the flexible nonmagnetic support, and a magnetic layer containing a ferromagnetic powder and a binder formed on the nonmagnetic layer, wherein the magnetic layer having an average thickness d of 0.01 xcexcm to 0.3 xcexcm, a ratio "sgr"/d of a standard deviation "sgr" of the thickness of the magnetic layer to the average thickness d equal to or less than 0.5, and, according to a roughness spectrum of the magnetic layer surface measured with an atomic force microscope, Power Spectrum Density of a roughness (PSD) of 0.5 nm2 or lower in a wavelength of 1 to 5 xcexcm , and PSD from 0.02 to 0.5 nm2 in a wavelength of 0.5 xcexcm or higher to 1 xcexcm or less.
In the magnetic recording medium according to the invention, it is preferable to set that a coercive force Hc in an in-plane direction of the magnetic layer is 1500 Oe or higher to 4000 Oe or less and that residual magnetization in the in-plane direction per 1 cm2 of the magnetic layer is 0.0005 emu to 0.005 emu.
In the magnetic recording medium according to the invention, it is preferable that a surface roughness spectrum, measured with an atomic force microscope, on the magnetic layer side of the flexible nonmagnetic support has a roughness component strength of 0.5 nm2 or lower in a wavelength of 1 xcexcm to 5 xcexcm and PSD of 0.02 nm2 to 0.5 nm2 in a wavelength of 0.5 xcexcm or higher to less than 1 xcexcm.
In the magnetic recording medium according to the invention, it is preferable that binder weight B(L) of the nonmagnetic layer is 5 parts or more to 25 parts or lower by weight, with respect to 100 parts by weight of nonmagnetic powders, and that the ratio of the B(L) to binder weight B(U) of the magnetic layer is 0.1 less than B(U)/B(L)xe2x89xa61.0.